Combine the oil and butter in a large saucepan. Warm over medium-low heat. Add the leeks and stirring frequently, allow them cook slowly for 10 minutes. Do not brown.
Stir in the cauliflower, potatoes, and stock. Season with the salt and pepper. Bring to a boil over high heat. Reduce the heat to medium, cover, and simmer for 20 minutes, or until the vegetables are very tender. Remove form the heat and allow to cool slightly.
Transfer to a blender or a food processor fitted with the metal blade. Process until smooth. Transfer to a clean saucepan and place over medium heat. Bring to a simmer. Taste and adjust the seasoning. If the soup is too thick, thin with the milk.
Pour an equal portion into each of 4 warm bowls. Lay a parsley leaf in the center.
Recipe courtesy of The French Culinary Institute's Salute to Healthy Cooking